U.S. patent number 10,115,992 [Application Number 15/648,063] was granted by the patent office on 2018-10-30 for electrode catalyst, gas diffusion electrode-forming composition, gas diffusion electrode, membrane electrode assembly, and fuel cell stack.
This patent grant is currently assigned to N.E. Chemcat Corporation. The grantee listed for this patent is N.E. CHEMCAT Corporation. Invention is credited to Hiroshi Igarashi, Tomoteru Mizusaki, Kiyotaka Nagamori, Yoko Nakamura, Yasuhiro Seki.
United States Patent |
10,115,992 |
Nagamori , et al. |
October 30, 2018 |
Electrode catalyst, gas diffusion electrode-forming composition,
gas diffusion electrode, membrane electrode assembly, and fuel cell
stack
Abstract
Provided is an electrode catalyst in which the contents of
chlorine (Cl) species and bromine (Br) species are reduced to a
predetermined level or lower, capable of exhibiting sufficient
catalyst performance. The electrode catalyst has a core-shell
structure including a support, a core part formed on the support
and a shell part formed to cover at least a part of the surface of
the core part. A concentration of bromine (Br) species of the
electrode catalyst as measured by X-ray fluorescence (XRF)
spectroscopy is 400 ppm or less, and a concentration of chlorine
(Cl) species of the electrode catalyst as measured by X-ray
fluorescence (XRF) spectroscopy is 900 ppm or less.
Inventors: |
Nagamori; Kiyotaka (Bando,
JP), Mizusaki; Tomoteru (Bando, JP),
Nakamura; Yoko (Bando, JP), Igarashi; Hiroshi
(Bando, JP), Seki; Yasuhiro (Bando, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
N.E. CHEMCAT Corporation |
Tokyo |
N/A |
JP |
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Assignee: |
N.E. Chemcat Corporation
(Tokyo, JP)
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Family
ID: |
58289126 |
Appl.
No.: |
15/648,063 |
Filed: |
July 12, 2017 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20170331135 A1 |
Nov 16, 2017 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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15542367 |
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PCT/JP2016/076280 |
Sep 7, 2016 |
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Foreign Application Priority Data
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Sep 18, 2015 [JP] |
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2015-185974 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01G
55/005 (20130101); H01M 8/1004 (20130101); H01M
4/9075 (20130101); H01M 4/92 (20130101); B01J
23/44 (20130101); H01M 4/9058 (20130101); H01M
4/925 (20130101); B01J 23/89 (20130101); H01M
4/86 (20130101); H01M 8/141 (20130101); H01M
8/10 (20130101); Y02E 60/50 (20130101); G01N
23/223 (20130101) |
Current International
Class: |
H01M
8/1004 (20160101); H01M 4/90 (20060101); H01M
4/92 (20060101); H01M 4/86 (20060101) |
References Cited
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|
Primary Examiner: Rhee; Jane J
Attorney, Agent or Firm: Troutman Sanders LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is continuation of U.S. patent application Ser.
No. 15/542,367, filed Jul. 7, 2017, which is a U.S. national phase
application under 35 U.S.C. .sctn. 371 of International Patent
Application No. PCT/JP2016/076280, filed on Sep. 7, 2016, and
claims benefit of priority to Japanese Patent Application No.
2015-185974, filed Sep. 18, 2015. The entire contents of these
applications are hereby incorporated by reference.
Claims
The invention claimed is:
1. An electrode catalyst having a core-shell structure comprising:
a support; a core part formed on the support; and a shell part
formed to cover at least a part of a surface of the core part,
wherein the shell part includes a single-layered structure formed
to cover at least a part of the surface of the core part, or a
two-layered structure including a first shell part formed to cover
at least a part of the surface of the core part, and a second shell
part formed to cover at least a part of the surface of the first
shell part, wherein in a case of the single-layered shell part, the
shell part comprises platinum (Pt), and the core part comprises
palladium (Pd), whilst in a case of the two-layered shell part, the
first shell part comprises palladium (Pd), and the second shell
part comprises platinum (Pt), wherein a concentration of bromine
(Br) species measured by X-ray fluorescence (XRF) spectroscopy, is
not greater than 400 ppm, and wherein a concentration of chlorine
(Cl) species measured by X-ray fluorescence (XRF) spectroscopy, is
less than 900 ppm.
2. The electrode catalyst according to claim 1, wherein the
concentration of bromine (Br) species is not greater than 300
ppm.
3. The electrode catalyst according to claim 2, wherein the
concentration of bromine (Br) species is not greater than 200
ppm.
4. The electrode catalyst according to claim 1, wherein the
concentration of chlorine (Cl) species is equal to or greater than
0 ppm.
5. The electrode catalyst according to claim 1, wherein the
concentration of chlorine (Cl) species is not less than 100
ppm.
6. The electrode catalyst according to claim 1, wherein, in a case
of the two-layered shell part, the core part contains one or more
metal elements other than noble metals as a main component(s).
7. A composition for forming a gas diffusion electrode, comprising
the electrode catalyst according to claim 1.
8. A gas diffusion electrode comprising the electrode catalyst
according to claim 1.
9. A membrane-electrode assembly (MEA) comprising the gas diffusion
electrode according to claim 8.
10. A fuel cell stack comprising the membrane-electrode assembly
(MEA) according to claim 9.
11. The electrode catalyst according to claim 2, wherein the
concentration of chlorine (Cl) species is equal to or greater than
0 ppm.
12. The electrode catalyst according to claim 3, wherein the
concentration of chlorine (Cl) species is equal to or greater than
0 ppm.
13. The electrode catalyst according to claim 2, wherein the
concentration of chlorine (Cl) species not less than 100 ppm.
14. The electrode catalyst according to claim 3, wherein the
concentration of chlorine (Cl) species not less than 100 ppm.
Description
TECHNICAL FIELD
This invention relates to an electrode catalyst. Also, this
invention relates to a composition for forming a gas diffusion
electrode, a gas diffusion electrode, a membrane-electrode
assembly, and a fuel cell stack that include the electrode
catalyst.
BACKGROUND
A so-called polymer electrolyte fuel cell (Polymer Electrolyte Fuel
Cell: hereinafter called "PEFC" as needed), has its operating
temperature of from a room temperature to about 80.degree. C. Also,
since PEFC makes it possible to employ inexpensive general-purpose
plastics, etc. for members constituting its fuel cell body, it is
possible to realize reduction in weight. Furthermore, PEFC makes it
possible to achieve thinning of a polymer electrolyte membrane,
enabling an electric resistance to be reduced, thereby enabling a
power loss to be reduced relatively easily. Due to PEFC having not
a few advantages as described above, it is applicable to a fuel
cell vehicle, a home cogeneration system, and the like.
As an electrode catalyst for PEFC, there has been proposed an
electrode catalyst in which platinum (Pt) or a platinum (Pt) alloy,
i.e., a component for the electrode catalyst, is supported on a
carbon serving as a support (for example, Japanese Patent
Application Publication No. 2011-3492, MATSUOKA et al.,
"Degradation of Polymer Electrolyte fuel cells under the existence
of anion species", J. Power Sources, 2008 May 1, Vol. 179 No. 2,
P.560-565).
Conventionally, there have been disclosed that, as for an electrode
catalyst for PEFC, if the content of chlorine contained in the
electrode catalyst is 100 ppm or more, it is not desirable as an
electrode catalyst (for example, Japanese Patent No. 4,286,499);
and that this is because if the content of chlorine contained in
the electrode catalyst is 100 ppm or more, it is impossible to
obtain a sufficient catalytic activity for the electrode catalyst
for fuel cells; and corrosion of its catalyst layer will occur,
thus shortening the life of the fuel cell.
Then, there is disclosed, as the catalyst component of the
electrode catalyst, a powder of platinum (Pt) or platinum (Pt)
alloy that contains less than 100 ppm of chlorine (for example,
Japanese Patent No. 4,286,499).
As for the preparation of the powder of platinum (Pt) or platinum
(Pt) alloy, there is disclosed the following method: forming a melt
which contains, as starting materials, a chlorine-free platinum
compound and a chlorine-free compound of alloying elements; heating
the melt up to a reaction temperature at which the platinum
compound and the compound of the alloying elements are thermally
decomposed to give an oxide; cooling the melt; and the melt is
dissolved in water and the resulting oxide or mixed oxides are
converted into a powder of platinum or platinum alloy by successive
reduction.
Further, there is disclosed a PEFC in which part of protons of an
acid group of an electrolyte contained in a catalyst layer of an
electrode of a membrane-electrode assembly is exchanged for a
phosphonium ion, defining a compound structure such that a counter
anion of the phosphonium contains no halogen elements (for example,
Japanese Patent No. 5,358,997). The reason, as is disclosed
therein. is that residues of the halogen elements in the electrode
cause a degradation in cell performance. Specifically, it is
described that the residues of a fluoride ion, a chloride ion, or a
bromide ion amongst halide ions in the electrode sometimes cause
degradation in cell performance, and particularly, the residues of
the chloride ion in the electrode poison the electrode catalyst,
and cause Pt serving as a catalyst to be eluted from a catalyst
layer as a complex ion such as PtCl.sub.4.sup.2-, PtCl.sub.6.sup.2-
to cause degradation of the cell performance.
Moreover, there is disclosed a method for producing core-shell
particles obtained by filtering a dispersion liquid having
core-shell particles dispersed in a solvent using ultrafilters or
the like, cleaning and substituting the same with a solvent as
necessary (for example, Japanese Patent No. 5,443,029).
Specifically, there is disclosed in a preparation process of a core
metal particles dispersion liquid, the core metal particles
dispersion liquid was cleaned until no Cl ions were detected.
Further, there is disclosed a method for producing carbon supported
core-shell catalyst fine particles which controls a deposition of a
shell metal material composing a shell part on a surface of a
carbon support (for example, Japanese Patent No. 5,672,752).
Furthermore, there is disclosed a method for producing a platinum
core-shell catalyst capable of directly depositing platinum on a
gold core particle (for example, Japanese Patent No. 5,660,603). In
production processes provided in these two methods for producing a
electrode catalyst (core-shell catalyst), it is disclosed that the
electrode catalyst (core-shell catalyst) is cleaned with extra pure
water.
However, the methods for producing an electrode catalyst
(core-shell catalyst) disclosed in the aforementioned patent
documents focus on chlorine only amongst halogens, and merely work
on reduction/removal of chlorine although it shows the findings
that residues of halogens reduce cell performance.
The applicant of the present patent application presents the
following publications as those that describe the aforementioned
inventions known to the public through publications.
SUMMARY
As mentioned above, from the viewpoint of improving catalytic
activity and lifetime as an electrode catalyst for PEFC, it is
important to reduce the content of halogen, particularly of
chlorine species contained in the catalyst.
However, the present inventors have found out that when a
core-shell catalyst is employed as an electrode catalyst for PEFC,
a sufficient catalyst performance cannot be obtained unless not
only the content of chlorine species but the content of bromine
species is reduced to a predetermined level or lower, and that
bromine species has a greater impact than chlorine species on
degradation of catalyst performance. Namely, in a case where a
core-shell catalyst is employed as an electrode catalyst for PEFC,
there has been room for improvement in the aforementioned
conventional techniques.
This invention has been made in view of such technical
circumstances, and it is an object of this invention to provide an
electrode catalyst having the contents of chlorine species and
bromine species reduced to predetermined levels or lower, enabling
the electrode catalyst to exhibit sufficient catalytic
performance.
Further, it is another object of this invention to provide a
composition for forming a gas diffusion electrode, a gas diffusion
electrode, a membrane-electrode assembly (MEA), and a fuel cell
stack that include the aforementioned electrode catalyst.
The present inventors, as a result of having performed intensive
studies, found out that it is possible to constitute an electrode
catalyst which exhibits a satisfactory catalyst performance (a
core-shell catalyst to be described later) by reducing the
concentration of chlorine (Cl) species contained in the electrode
catalyst to 900 ppm or lower and by reducing the concentration of
bromine (Br) species contained therein to 400 ppm or lower, when
measured by X-ray fluorescence (XRF), and have completed this
invention.
More specifically, this invention includes the following technical
matters:
That is, this invention provides:
(1) an electrode catalyst having a core-shell structure including:
a support; a core part formed on the support; and a shell part
formed to cover at least a part of a surface of the core part,
wherein the concentration of bromine (Br) species is not higher
than 400 ppm when measured by X-ray fluorescence (XRF)
spectroscopy, and the concentration of chlorine (Cl) species is not
higher than 900 ppm when measured by X-ray fluorescence (XRF)
spectroscopy.
Since the concentrations of chlorine (Cl) species and bromine (Br)
species contained in the catalyst are respectively rendered to be
not greater than 900 ppm and not greater than 400 ppm, the
electrode catalyst of this invention can exhibit a sufficient
catalytic activity as an electrode catalyst.
Further, the electrode catalyst has a core-shell structure, and is
suitable for reducing the manufacturing cost.
In this invention, the bromine (Br) species refers to a chemical
species containing bromine as a constituent element. Specifically,
the chemical species containing bromine include bromine atom (Br),
bromine molecule (Br.sub.2), bromide ion (Br.sup.-), bromine
radical (Br.), polyatomic bromine ion and a bromine compound (e.g.
X--Br where X represents a counterion).
In this invention, the chlorine (Cl) species refers to a chemical
species containing chlorine as a constituent element. Specifically,
the chemical species containing chlorine include chlorine atom
(Cl), chlorine molecule (Cl.sub.2), chloride ion (Cl.sup.-),
chlorine radical (Cl.), polyatomic chloride ion and a chlorine
compound (e.g. X--Cl where X represents a counterion).
In this invention, bromine (Br) species concentration and chlorine
(Cl) species concentration are measured by X-ray fluorescence (XRF)
spectrometry. A value of the bromine (Br) species contained in the
electrode catalyst that is measured by X-ray fluorescence (XRF)
spectrometry is the concentration of bromine (Br) species.
Likewise, A value of the chlorine (Cl) species contained in the
electrode catalyst that is measured by X-ray fluorescence (XRF)
spectrometry is the concentration of chlorine (Cl) species.
Here, the bromine (Br) species concentration and chlorine (Cl)
species concentration are concentrations of the bromine atoms and
chlorine atoms in terms of the bromine element and chlorine element
that are respectively contained in the electrode catalyst.
As stated above, with respect to the core-shell catalyst, the
present inventors also focused on bromine (Br) species other than
chlorine (Cl) species, and found out that it is important to
sufficiently remove them as impurities.
Bromine being a halogen element as with chlorine, is an element
from the same family (7B) as chlorine, and they have analogous
physical properties as represented by e.g., their ion radii. For
this reason, a bromine-containing metal compound is sometimes used
as a raw material for the core part or shell part of a core-shell
catalyst, instead of a chloride salt of palladium (Pd) and platinum
(Pt). Further, the bromine-containing metal compound is sometimes
used as a precursor for producing the chloride salt of platinum
(Pt) that is employed as a raw material of the core-shell catalyst.
Moreover, during a production process, the bromine (Br) species is
sometimes unintentionally mixed with the core-shell catalyst,
adhering to the electrode catalyst as an impurity.
Further, the present inventors found out that the bromine (Br)
species has a greater impact on the degradation of catalyst
performance, than the chloride (Cl) species. From this viewpoint,
in the electrode catalyst of this invention as set forth in
(1),
(2) the bromine (Br) species concentration is preferably not
greater than 300 ppm, and
(3) the bromine (Br) species concentration is more preferably not
greater than 200 ppm.
Thus can be achieved the effects of this invention more
reliably.
Further, from the viewpoint of more reliably achieving the effects
of this invention, it is preferable to reduce as much chlorine (Cl)
species as possible, specifically,
(4) the chlorine (Cl) species concentration is preferably less than
900 ppm, more preferably not greater than 800 ppm, and still more
preferably not greater than 600 ppm.
Further, in this invention,
(5) the chlorine (Cl) species concentration may be equal to or
higher than 0 ppm. Here, in this invention, "the chlorine (Cl)
species concentration is 0 ppm" denotes a state in which the
chloride (Cl) species measured through X-ray fluorescence (XRF)
spectroscopy, is reduced to a level at which the chloride (Cl)
species is not detected (undetected). Although it is ideal that the
chlorine (Cl) species is thoroughly removed in order to improve the
catalyst performance, the present inventors confirmed that the
catalyst performance sufficiently improves by reducing the chloride
(Cl) species to a level at which the chloride (Cl) species measured
through X-ray fluorescence (XRF) spectroscopy, is not detected
(undetected). In the case of this invention, the detection limit of
the chlorine (Cl) species measured through X-ray fluorescence (XRF)
spectroscopy, is 100 ppm. Consequently, in the case that "the
chlorine (Cl) species concentration is 0 ppm", there is a
possibility that the chlorine (Cl) species may be contained in a
concentration range of less than 100 ppm.
Further, the bromine (Br) species concentration may be equal to or
higher than 0 ppm. Similarly, with regard to the bromine (Br)
species concentration, "the bromine (Br) species concentration is 0
ppm" denotes a state in which the bromine (Br) species measured
through X-ray fluorescence (XRF) spectroscopy, is reduced to a
level at which the bromine (Br) species is not detected
(undetected). As to this bromine (Br) species concentration as
well, the present inventors confirmed that the catalyst performance
has sufficiently improved by reducing the bromine (br) species to a
level at which the bromine (Br) species measured through X-ray
fluorescence (XRF) spectroscopy, is not detected (undetected). In
the case of this invention, the detection limit of the bromine (Br)
species measured through X-ray fluorescence (XRF) spectroscopy, is
100 ppm. Consequently, in the case that "the bromine (Br) species
concentration is 0 ppm", there is a possibility that the bromine
(Br) species may be contained in a concentration range of less than
100 ppm.
Further, in the electrode catalysts (1) to (5) of this invention,
if the bromine (Br) species and the chlorine (Cl) species
concentrations have been respectively reduced to not greater than
400 ppm and not greater than 900 ppm,
(6) the chlorine (Cl) species concentration may be not less than
100 ppm. Namely, the chlorine (Cl) species concentration may be
from 100 ppm to 900 ppm.
In this case as well, the effects of this invention can be
attained. According to this structure, the chlorine (Cl) species
concentration is not reduced to less than 100 ppm, thus making it
possible to reduce costs and labors for reducing the chlorine in
the production process.
Further, this invention provides
(7) the electrode catalyst set forth in any one of (1) to (6),
wherein the core-shell structure includes: the core part; and a
single-layered shell part having the shell part formed to cover at
least a part of the surface of the core part.
In this case as well, the effects of this invention can be
attained. By employing the aforementioned structure, the electrode
catalyst of this invention may reduce the content of a noble metal
(s) such as platinum used in the core part, thereby enabling
reduction in raw material cost.
According to the structure of the invention (7), namely,
(8) in a case of the shell part being of the single-layered
structure, it is preferable that the shell part contain at least
one metal selected from platinum (Pt) and a platinum (Pt) alloy.
This makes it possible to more easily obtain a superior catalytic
activity.
Further, according to the structure of the invention (8),
namely,
(9) in a case of the shell part being of the single-layered
structure, it is preferable that the core part contain at least one
kind of metal selected from the group consisting of palladium (Pd),
a palladium (Pd) alloy, a platinum (Pt) alloy, gold (Au), nickel
(Ni) and a nickel (Ni) alloy. This makes it possible to more
reliably obtain the effects of this invention. Furthermore, by
employing the aforementioned structure, a higher catalyst activity
and a higher durability can be obtained.
Further, when this invention has the structure (8), namely,
(10) in a case of the shell part being of the single-layered
structure, the core part may contain one or more metal elements
other than noble metals as a main component(s). In this structure
as well, the effects of this invention can be attained.
Furthermore, by employing this structure, cost reduction can be
easily achieved due to reduction in noble metal content.
Further, this invention provides the electrode catalyst as set
forth in any one of (7) to (9), in which,
(11) in a case of the shell part being of the single-layered
structure, the support contains an electrically conductive carbon,
the shell part contains platinum (Pt), and the core part contains
palladium (Pd).
Thus, the effects of this invention can be achieved more reliably.
Further, by employing the abovementioned structure, there can be
achieved a higher catalytic activity and a higher durability.
Furthermore, by employing the abovementioned structure, the
electrode catalyst of this invention, as compared to conventional
electrode catalysts having a structure where platinum is supported
on a carbon support, is capable of reducing the amount of platinum
contained, and is thus capable of easily reducing a raw material
cost.
Furthermore, according to this invention,
(12) there is provided the electrode catalyst as set forth in any
one of (1) to (6),
in which the core-shell structure has:
the core part; and
a two-layered shell part having a first and a second shell parts
formed such that the first shell part covers at least a part of the
surface of the core part, and the second shell part covers at least
a part of a surface of the first shell part.
Thus, the effects of this invention can be achieved more reliably.
By employing the abovementioned structure, the electrode catalyst
of this invention may reduce the contained amount of a noble
metal(s) such as platinum used in the core part, and is thus
capable of easily reducing a raw material cost.
When this invention employs the structure of the invention (12),
namely,
(13) in a case of the shell part being of the two-layered
structure, it is preferable that the second shell part contain at
least one metal selected from platinum (Pt) and a platinum (Pt)
alloy. This makes it possible to more easily obtain a superior
catalytic activity.
Further, when this invention employs the structure of the invention
(13), namely,
(14) in a case of the shell part being of the two-layered
structure, it is preferable that the first shell part contain at
least one kind of metal selected from the group consisting of
palladium (Pd), a palladium (Pd) alloy, a platinum (Pt) alloy, gold
(Au), nickel (Ni) and a nickel (Ni) alloy. In this way, the effects
of this invention can be achieved more reliably. Furthermore,
employing the aforementioned structure makes it possible to obtain
a higher catalytic activity and a higher durability.
Further, when this invention has the structure (14), namely,
(15) in a case of the shell part being of the two-layered
structure, it is preferable that the core part contain one or more
metal elements other than noble metals as a main component(s). In
this structure as well, the effects of this invention can be
attained. Furthermore, by employing this structure, cost reduction
can be more easily achieved due to reduction in noble metal
content.
Also, this invention provides the electrode catalyst as set forth
in (13) to (15) in which
(16) in a case of the shell part being of the two-layered
structure, the first shell part contains palladium (Pd), and the
second shell part contains platinum (Pt).
In this way, the effects of this invention can be achieved more
reliably. By employing the abovementioned structure, there can be
achieved a higher catalytic activity and a higher durability.
Further, this invention provides
(17) a composition for forming a gas diffusion electrode, including
the electrode catalyst as set forth in any one of (1) to (16).
According to the gas diffusion electrode-forming composition of
this invention, it is possible to easily produce a gas diffusion
electrode with a high catalytic activity (polarization property)
because it contains the electrode catalyst of this invention.
Furthermore, this invention provides
(18) a gas diffusion electrode containing the electrode catalyst as
set forth in any one of (1) to (16).
According to the gas diffusion electrode of this invention, it is
possible to achieve a high catalytic activity (polarization
property) because it contains the electrode catalyst of this
invention.
Furthermore, this invention provides
(19) a membrane-electrode assembly (MEA) including the gas
diffusion electrode as set forth in (18).
According to the membrane-electrode assembly (MEA) of this
invention, it is possible to achieve a high battery property
because it contains the gas diffusion electrode of this
invention.
Still further, this invention provides
(20) a fuel cell stack including the membrane-electrode assembly
(MEA) as set forth in (19).
According to the fuel cell stack of this invention, it is possible
to achieve a high battery property because it contains the
membrane-electrode assembly (MEA) of this invention.
According to this invention, there can be provided an electrode
catalyst that can exhibit a sufficient catalytic activity, because
the concentrations of chlorine (Cl) species and bromine (Br)
species contained in the electrode catalyst are respectively
rendered to be not greater than 900 ppm (preferably less than 900
ppm) and not greater than 400 ppm (preferably not greater than 300
ppm, more preferably not greater than 200 ppm).
Further, according to this invention, there can be provided an
electrode catalyst that is also suitable for reduction of the
manufacturing cost because the electrode catalyst has the
core-shell structure.
Further, according to this invention, there can be provided a
composition for forming a gas diffusion electrode, a gas diffusion
electrode, a membrane-electrode assembly (MEA), and a fuel cell
stack that include the aforementioned electrode catalyst.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic sectional view showing an example of the
electrode catalyst (core-shell catalyst) of this invention.
FIG. 2 is a schematic sectional view showing another example of the
electrode catalyst (core-shell catalyst) of this invention.
FIG. 3 is a schematic sectional view showing another example of the
electrode catalyst (core-shell catalyst) of this invention.
FIG. 4 is a schematic sectional view showing another example of the
electrode catalyst (core-shell catalyst) of this invention.
FIG. 5A is a schematic diagram showing an example of a fuel cell
stack of this invention.
FIG. 5B is a magnified portion of FIG. 5A illustrating the gas
diffusion layer and an electrode catalyst layer of the present
invention.
FIG. 6 is a schematic diagram showing a schematic configuration of
a rotating disk electrode measuring device equipped with a rotating
disc electrode used in a working example.
DETAILED DESCRIPTION
Examples of this invention are described in detail hereunder with
reference to the drawings when necessary.
FIG. 1 is a schematic cross-sectional view showing a preferable
embodiment of an electrode catalyst (core-shell catalyst) of this
invention.
As shown in FIG. 1, an electrode catalyst 1 of this invention
includes a support 2; and catalyst particles 3 supported on the
support 2 and having a so-called "core-shell structure." Each
catalyst particle 3 has a core part 4; and a shell part 5 covering
at least a part of the surface of the core part 4. The catalyst
particles 3 thus have a so-called "core-shell structure" including
the core part 4 and the shell part 5 formed on the core part 4.
That is, the electrode catalyst 1 has the catalyst particles 3
supported on the support 2, and the catalyst particles 3 have the
structure where the core part 4 serves as a core (core portion),
and the shell part 5 as a shell covers at least a part of the
surface of the core part 4.
Further, the constituent element (chemical composition) of the core
part 4 and the constituent element (chemical composition) of the
shell part 5 differ from each other in composition.
There are no particular restrictions on the electrode catalyst 1 of
this invention except that the shell part 5 has to be formed on at
least a part of the surface of the core part 4 of each catalyst
particle 3.
For example, in terms of more reliably achieving the effects of
this invention, it is preferred that the electrode catalyst 1 be in
a state where the whole range of the surface of the core part 4 is
substantially covered by the shell part 5, as shown in FIG. 1.
Further, the electrode catalyst 1 may also be in a state where a
part of the surface of the core part 4 is covered by the shell part
5, and the rest part of the surface of the core part 4 is thus
partially exposed, provided that the effects of this invention can
be achieved.
That is, with regard to the electrode catalyst of this invention,
it is sufficient that the shell part be formed on at least a part
of the surface of the core part.
FIG. 2 is a schematic cross-sectional view showing another
preferable embodiment (electrode catalyst 1A) of the electrode
catalyst (core-shell catalyst) of this invention.
As shown in FIG. 2, an electrode catalyst 1A of this invention has
catalyst particles 3a each being composed of a core part 4; a shell
part 5a covering a part of the surface of the core part 4; and a
shell part 5b covering an other part of the surface of the core
part 4.
With regard to the catalyst particles 3a contained in the electrode
catalyst 1A shown in FIG. 2, there is a part of the core part 4
that is neither covered by the shell part 5a nor covered by the
shell part 5b. This part of the core part 4 composes a core
part-exposed surface 4s.
That is, as shown in FIG. 2, so far as the effects of this
invention can be achieved, the catalyst particles 3a contained in
the electrode catalyst 1A may also be in a state where the surface
of the core part 4 is partially exposed (e.g. a state where 4s as a
part of the surface of the core part 4 shown in FIG. 2 is
exposed).
In other words, as is the case with the electrode catalyst 1A shown
in FIG. 2, the shell part 5a may be partially formed on a part of
the surface of the core part 4, and the shell part 5b may then be
partially formed on another part of the surface of the core part
4.
FIG. 3 is a schematic cross-sectional view showing another
preferable embodiment (electrode catalyst 1B) of the electrode
catalyst (core-shell catalyst) of this invention.
As shown in FIG. 3, an electrode catalyst 1B of this invention has
catalyst particles 3 each being composed of a core part 4; and a
shell part 5 substantially covering the whole range of the surface
of the core part 4.
The shell part 5 may have a two-layered structure composed of a
first shell part 6 and a second shell part 7. That is, the catalyst
particles 3 have a so-called "core-shell structure" comprised of
the core part 4; and the shell part 5 (first shell part 6 and
second shell part 7) formed on the core part 4.
The electrode catalyst 1B has a structure where the catalyst
particles 3 are supported on the support 2, having the core part 4
serving as a core (core portion); and the whole range of the
surface of the core part 4 is substantially covered by the shell
part 5 composed of the first shell part 6 and the second shell part
7.
Here, the constituent element (chemical composition) of the core
part 4, the constituent element (chemical composition) of the first
shell part 6 and the constituent element (chemical composition) of
the second shell part 7 differ from one another in composition.
Moreover, the shell part 5 included in the electrode catalyst 1B of
this invention may further include another shell part in addition
to the first shell part 6 and the second shell part 7.
In terms of more reliably achieving the effects of this invention,
it is preferred that the electrode catalyst 1B be in a state where
the whole range of the surface of the core part 4 is substantially
covered by the shell part 5, as shown in FIG. 3.
FIG. 4 is a schematic cross-sectional view showing another
preferable embodiment (electrode catalyst 1C) of the electrode
catalyst (core-shell catalyst) of this invention.
As shown in FIG. 4, an electrode catalyst 1C of this invention has
catalyst particles 3a each being composed of a core part 4; a shell
part 5a covering a part of the surface of the core part 4; and a
shell part 5b covering another part of the surface of the core part
4.
The shell part 5a may have a two-layered structure composed of a
first shell part 6a and a second shell part 7a.
Further, the shell part 5b may have a two-layered structure
composed of a first shell part 6b and a second shell part 7b.
That is, the catalyst particles 3a have a so-called "core-shell
structure" including the core part 4; the shell part 5a (first
shell part 6a and second shell part 7a) formed on the core part 4;
and the shell part 5b (first shell part 6b and second shell part
7b) formed on the core part 4.
With regard to the shell part 5b composing the catalyst particle 3a
shown in FIG. 4, there is a part of the first shell part 6b that is
not covered by the second shell part 7b. The part of the first
shell part 6b that is not covered by the second shell part 7b
composes a first shell part-exposed surface 6s.
With regard to the shell part 5a composing the catalyst particle 3a
shown in FIG. 4, it is preferred that the whole range of the first
shell part 6a be substantially covered by the second shell part
7a.
Further, as shown in FIG. 4 and with regard to the shell part 5b
composing each catalyst particle 3a, also permissible is a state
where a part of the surface of the first shell part 6b is covered,
and the surface of the first shell part 6b is thus partially
exposed (e.g. a state shown in FIG. 4 where the part 6s of the
surface of the first shell part 6b is exposed), so far as the
effects of this invention can be achieved.
Moreover, on the premise that the effects of this invention can be
achieved, the electrode catalyst 1 may allow a "complex of the core
part 4 and shell part 5 with the whole range of the surface of the
core part 4 being substantially covered by the shell part 5" and a
"complex of the core part 4 and shell part 5 with the surface of
the core part 4 being partially covered by the shell part 5" to
coexist on the support 2 in a mixed manner.
Specifically, the electrode catalyst of this invention may be in a
state where the electrode catalysts 1 and 1A shown in FIGS. 1 and 2
and the electrode catalysts 1B and 1C shown in FIGS. 3 and 4
coexist in a mixed manner, provided the effects of this invention
can be achieved.
Further, the electrode catalyst of this invention may allow the
shell part 5a and the shell part 5b to coexist in a mixed manner
with respect to an identical core part 4, as shown in FIG. 4, so
far as the effects of this invention can be achieved.
Furthermore, so far as the effects of this invention can be
achieved, the electrode catalyst of this invention may allow only
the shell part 5a to exist with respect to an identical core part 4
or only the shell part 5b to exist with respect to an identical
core part 4 (none of these states are shown in the drawings).
Furthermore, so far as the effects of this invention can be
achieved, the electrode catalyst 1 may also be in a state where
"particles only composed of the core parts 4 that are not covered
by the shell parts 5" are supported on the support 2, in addition
to at least one kind of the electrode catalysts 1, 1A, 1B and 1C
(not shown).
Furthermore, on the premise that the effects of this invention can
be achieved, the electrode catalyst 1 may also be in a state where
"particles only composed of the constituent element of the shell
part 5" are supported on the support 2 without being in contact
with the core parts 4, in addition to at least one kind of the
electrode catalysts 1, 1A, 1B and 1C (not shown).
Furthermore, on the premise that the effects of this invention can
be achieved, the electrode catalyst 1 may also be in a state where
"particles only composed of the core parts 4 that are not covered
by the shell parts 5" and "particles only composed of the
constituent element of the shell part 5" are individually and
independently supported on the support 2, in addition to at least
one kind of the electrode catalysts 1, 1A, 1B and 1C.
It is preferred that the core part 4 have an average particle
diameter of 2 to 40 nm, more preferably 4 to 20 nm, particularly
preferably 5 to 15 nm.
As for the thickness of the shell part 5 (thickness from the
surface in contact with the core part 4 to the outer surface of the
shell part 5), a preferable range thereof is to be appropriately
determined based on the design concept(s) of the electrode
catalyst.
For example, when the amount of the metal element (e.g. platinum)
used to compose the shell part 5 is intended to be minimized, a
layer composed of one atom (one atomic layer) is preferred. In this
case, when there is only one kind of metal element composing the
shell part 5, it is preferred that the thickness of the shell part
5 be twice as large as the diameter of one atom of such metal
element (in spherical approximation). Further, when there are not
fewer than two kinds of metal elements composing the shell part 5,
it is preferred that the thickness of the shell part 5 be that of a
layer of one atom (one atomic layer formed with two or more kinds
of atoms being apposed on the surface of the core part 4).
Further, for example, when attempting to improve a durability by
employing a shell part 5 of a larger thickness, it is preferred
that such thickness be 1 to 10 nm, more preferably 2 to 5 nm.
When the shell part 5 has the two-layered structure composed of the
first shell part 6 and the second shell part 7, preferable ranges
of the thicknesses of the first shell part 6 and second shell part
7 are appropriately determined based on the design concept(s) of
the electrode catalyst of this invention.
For example, when the amount of a noble metal such as platinum (Pt)
as a metal element contained in the second shell part 7 is intended
to be minimized, it is preferred that the second shell part 7 be a
layer composed of one atom (one atomic layer). In this case, when
there is only one kind of metal element composing the second shell
part 7, it is preferred that the thickness of the second shell part
7 be approximately twice as large as the diameter of one atom of
such metal element (provided that an atom is considered as a
sphere).
Further, when there are not fewer than two kinds of metal elements
contained in the second shell part 7, it is preferred that the
second shell part 7 have a thickness equivalent to that of a layer
composed of not fewer than one kind of atom (one atomic layer
formed with two or more kinds of atoms being apposed in the surface
direction of the core part 4). For example, when attempting to
improve the durability of the electrode catalyst by employing a
second shell part 7 of a larger thickness, it is preferred that the
thickness of the second shell part 7 be 1.0 to 5.0 nm. If the
durability of the electrode catalyst is to be further improved, it
is preferred that the thickness of the second shell part 7 be 2.0
to 10.0 nm.
Here, in this invention, "average particle diameter" refers to an
average value of the diameters of an arbitrary number of particles
as particle groups that are observed through electron
micrographs.
There are no particular restrictions on the support 2, as long as
such support 2 is capable of supporting the catalyst particles 3 as
the complexes composed of the core parts 4 and the shell parts 5,
and has a large surface area.
Moreover, it is preferred that the support 2 be that exhibiting a
favorable dispersibility and a superior electrical conductivity in
a composition used to form a gas diffusion electrode having the
electrode catalyst 1.
The support 2 may be appropriately selected from carbon-based
materials such as glassy carbon (GC), fine carbon, carbon black,
black lead, carbon fiber, activated carbon, ground product of
activated carbon, carbon nanofiber and carbon nanotube; and
glass-based or ceramic-based materials such as oxides.
Among these materials, carbon-based materials are preferred in
terms of their adsorptivities with respect to the core part 4 and
in terms of a BET specific surface area of the support 2.
Further, as a carbon-based material, an electrically conductive
carbon is preferred. Particularly, an electrically conductive
carbon black is preferred as an electrically conductive carbon.
Examples of such electrically conductive carbon black include
products by the names of "Ketjenblack EC300 J," "Ketjenblack EC600"
and "Carbon EPC" (produced by Lion Corporation).
There are no particular restrictions on the component of the core
part 4, as long as the component is capable of being covered by the
shell part 5.
When the shell part 5 employs a single-layered structure as are the
cases with the electrode catalysts 1 and 1A that are shown in FIGS.
1 and 2 instead of the two-layered structure, from the viewpoint of
relatively easily obtaining a superior catalytic activity, it is
preferable that the core part 4 include a noble metal(s) as a main
component(s). The core part 4 composing the catalyst particles 3
and 3a of the electrode catalysts 1 and 1A, contains at least one
kind of metal selected from the group consisting of palladium (Pd),
a palladium (Pd) alloy, a platinum (Pt) alloy, gold (Au), nickel
(Ni) and a nickel (Ni) alloy.
There are no particular restrictions on a palladium (Pd) alloy, as
long as the alloy is to be obtained by combining palladium (Pd)
with another metal capable of forming an alloy when combined with
palladium (Pd). For example, such palladium (Pd) alloy may be a
two-component palladium (Pd) alloy obtained by combining palladium
(Pd) with another metal; or a three or more-component palladium
(Pd) alloy obtained by combining palladium (Pd) with not fewer than
two kinds of other metals. Specifically, examples of such
two-component palladium (Pd) alloy include gold palladium (PdAu),
silver palladium (PdAg) and copper palladium (PdCu). One example of
a three-component palladium (Pd) alloy is gold-silver-palladium
(PdAuAg).
There are no particular restrictions on a platinum (Pt) alloy, as
long as the alloy is to be obtained by combining platinum (Pt) with
another metal capable of forming an alloy when combined with
platinum (Pt). For example, such platinum (Pt) alloy may be a
two-component platinum (Pt) alloy obtained by combining platinum
(Pt) with another metal; or a three or more-component platinum (Pt)
alloy obtained by combining platinum (Pt) with not fewer than two
kinds of other metals. Specifically, examples of such two-component
platinum (Pt) alloy include nickel platinum (PtNi) and cobalt
platinum (PtCo).
There are no particular restrictions on a nickel (Ni) alloy, as
long as the alloy is to be obtained by combining nickel (Ni) with
another metal capable of forming an alloy when combined with nickel
(Ni). For example, such nickel (Ni) alloy may be a two-component
nickel (Ni) alloy obtained by combining nickel (Ni) with another
metal; or a three or more-component nickel (Ni) alloy obtained by
combining nickel (Ni) with not fewer than two kinds of other
metals. Specifically, one example of such two-component nickel (Ni)
alloy is tungsten nickel (NiW).
The shell part 5 contains at least one kind of metal selected from
platinum (Pt) and a platinum (Pt) alloy. There are no particular
restrictions on a platinum (Pt) alloy, as long as the alloy is to
be obtained by combining platinum (Pt) with an other metal capable
of forming an alloy when combined with platinum (Pt). For example,
such platinum (Pt) alloy may be a two-component platinum (Pt) alloy
obtained by combining platinum (Pt) with an other metal; or a three
or more-component platinum (Pt) alloy obtained by combining
platinum (Pt) with not fewer than two kinds of other metals.
Specifically, examples of such two-component platinum (Pt) alloy
include nickel platinum (PtNi), cobalt platinum (PtCo), platinum
ruthenium (PtRu), platinum molybdenum (PtMo) and platinum titanium
(PtTi). Particularly, in order for the shell part 5 to have a
poisoning resistance against carbon monoxide, it is preferred that
a platinum ruthenium (PtRu) alloy be used.
Further, when the shell part 5 employs the single-layered structure
as are the cases with the electrode catalysts 1 and 1A that are
shown in FIGS. 1 and 2 instead of the two-layered structure, from
the perspective of reducing the cost for producing the electrode
catalyst 1, it is preferred that the core part 4 include a metal
element(s) other than noble metals as a main component(s) (the
amount of the main component(s) is preferably not less than 60% by
weight, more preferably not less than 70% by weight, further
preferably not less than 80% by weight of the core part 4).
Specifically, when the shell part 5 employs the single-layered
structure, it is preferred that the core part 4 contain, as a main
component(s) thereof, a metal(s) including a metal element(s) other
than noble metals, a metal nitride of such metal, a metal carbide
of such metal, a metal oxide of such metal, an alloy containing
such metal (a solid solution containing such metal and an
intermetallic compound containing such metal), and/or a mixture of
such metal(s) and such metal compound (s) (the amount of the main
component(s) is preferably not less than 60% by weight, more
preferably not less than 70% by weight, and further preferably not
less than 80% by weight of the core part 4). In this case, it is
preferred that the metal elements other than noble metals be metal
elements other than Pt, Pd, Au, Ag, Rh, Ir, Ru and Os.
Further, in this case, it is preferred that the metal nitride be at
least one kind selected from the group of Ti nitride, Zr nitride,
Ta nitride, Nb nitride and W nitride.
Moreover, in this case, it is preferred that the metal carbide be
at least one kind selected from the group of Ti carbide, Zr
carbide, Ta carbide, Nb carbide and W carbide.
Furthermore, in this case, it is preferred that the metal oxide be
at least one kind selected from the group of Ti oxide, Zr oxide, Ta
oxide, Nb oxide and W oxide.
Further, as the electrode catalysts 1B and 1C illustrated in FIGS.
3 and 4, when the shell part 5 employs the two-layered structure
composed of the first shell part 6 and the second shell part 7, it
is preferred, especially from the perspective of reducing the cost
for producing the electrode catalyst 1, that the core part 4
contain a metal element(s) other than noble metals as a main
component(s) (the amount of main component is preferably not less
than 60% by weight, more preferably not less than 70% by weight,
further preferably not less than 80% by weight of the core part
4).
Specifically, when the shell part 5 employs the two-layered
structure, it is preferred that the core part 4 contain, as a main
component(s) thereof, a metal(s) including a metal element(s) other
than noble metals, a metal nitride of such metal, a metal carbide
of such metal, a metal oxide of such metal, an alloy containing
such metal (a solid solution containing such metal and an
intermetallic compound containing such metal), and/or a mixture of
such metal(s) and such metal compound (the amount of the main
component(s) is preferably not less than 60% by weight, more
preferably not less than 70% by weight, and further preferably not
less than 80% by weight of the core part 4). In this case, it is
preferred that the metal elements other than noble metals be metal
elements other than Pt, Pd, Au, Ag, Rh, Ir, Ru and Os.
Further, in this case, it is preferred that the metal nitride be at
least one kind selected from the group of Ti nitride, Zr nitride,
Ta nitride, Nb nitride and W nitride.
Moreover, in this case, it is preferred that the metal carbide be
at least one kind selected from the group of Ti carbide, Zr
carbide, Ta carbide, Nb carbide and W carbide.
Furthermore, in this case, it is preferred that the metal oxide be
at least one kind selected from the group of Ti oxide, Zr oxide, Ta
oxide, Nb oxide and W oxide.
A supported amount of the platinum (Pt) contained in the shell part
5 is 5 to 30% by weight, preferably 8 to 25% by weight with respect
to the weight of the electrode catalyst 1. It is preferred that the
amount of the platinum (Pt) supported be not smaller than 5% by
weight, because the electrode catalyst can fully exert its
catalytic activity in such case. It is also preferred that the
amount of the platinum (Pt) supported be not larger than 30% by
weight, because the amount of platinum (Pt) used is thus reduced in
such case, which is favorable in terms of production cost.
In the case where the shell part 5 has the two-layered structure
composed of the first shell part 6 and the second shell part 7, it
is preferred that the first shell part 6 contain at least one kind
of metal selected from the group consisting of palladium (Pd), a
palladium (Pd) alloy, a platinum (Pt) alloy, gold (Au), nickel (Ni)
and a nickel (Ni) alloy, and it is more preferred that the first
shell part 6 contain elemental palladium (Pd).
From the perspective of further improving the catalytic activities
of the electrode catalysts 1B and 1C and more easily obtaining the
same, it is preferred that the first shell part 6 be mainly
composed of palladium (Pd) simple substance (not less than 50 wt
%), and it is more preferred that such first shell part 6 be only
composed of palladium (Pd) simple substance.
It is preferred that the second shell part 7 contain at least one
kind of metal selected from platinum (Pt) and a platinum (Pt)
alloy, and it is more preferred that such shell part 7 contain
platinum (Pt) simple substance.
From the perspective of further improving the catalytic activities
of the electrode catalysts 1B and 1C and more easily obtaining the
same, it is preferred that the second shell part 7 be mainly
composed of platinum (Pt) simple substance (not less than 50 wt %),
and it is more preferred that such second shell part 7 be only
composed of platinum (Pt) simple substance.
Concentration of bromine (Br) species and concentration of chlorine
(Cl) species The electrode catalyst 1 exhibits a bromine (Br)
species concentration of not greater than 400 ppm (0 to 400 ppm),
preferably not greater than 300 ppm (0 to 300 ppm), more preferably
not greater than 200 ppm (0 to 200 ppm) when measured through X-ray
fluorescence (XRF) spectroscopy. Further, the electrode catalyst 1
satisfies the abovementioned conditions of the bromine (Br) species
concentration, and a chlorine (Cl) species concentration of not
greater than 900 ppm (0 to 900 ppm) when measured through the same
analytical method. The chlorine (Cl) species concentration is
preferably less than 900 ppm (not less than 0 ppm and not greater
than 900 ppm), more preferably not greater than 800 ppm (0 to 800
ppm), further preferably not greater than 600 ppm (0 to 600
ppm).
The electrode catalyst 1 is capable of fully exerting its catalytic
activity as an electrode catalyst by concurrently fulfilling the
abovementioned conditions of the chloride (Cl) species
concentration and the bromine (Br) species concentration.
Here, the bromine (Br) species concentration and the chlorine (Cl)
species concentration are measured through X-ray fluorescence (XRF)
spectroscopy. A value obtained by measuring the bromine (Br)
species contained in the electrode catalyst through X-ray
fluorescence (XRF) spectroscopy is the bromine (Br) species
concentration. Similarly, a value obtained by measuring the
chlorine (Cl) species contained in the electrode catalyst through
X-ray fluorescence (XRF) spectroscopy is the chlorine (Cl) species
concentration.
Here, the bromine (Br) species concentration and the chlorine (Cl)
species concentration are respectively the concentrations of the
bromine atoms and chlorine atoms in terms of the bromine and
chlorine elements contained in the electrode catalyst.
X-ray fluorescence (XRF) spectroscopy is a method where a specimen
containing a particular element A is irradiated with a primary
X-ray to generate a fluorescent X-ray of such element A, followed
by measuring the intensity of such fluorescent X-ray of the element
A such that quantitative analysis of the captioned element A
contained in the specimen can be performed. When performing
quantitative analysis through X-ray fluorescence (XRF)
spectroscopy, there may be employed the fundamental parameter
method (FP method) used in theoretical operation.
The FP method applies the idea that if the compositions and kinds
of the elements contained in a specimen are all known, the
fluorescent X-ray (XRF) intensities thereof can be individually and
theoretically calculated. In addition, the FP method allows there
to be estimated a composition(s) corresponding to the fluorescent
X-ray (XRF) of each element that is obtained by measuring the
specimen.
X-ray fluorescence (XRF) spectroscopy is performed using general
fluorescent X-ray (XRF) analyzers such as an energy dispersive
fluorescent X-ray (XRF) analyzer, a scanning-type fluorescent X-ray
(XRF) analyzer and a multi-element simultaneous-type fluorescent
X-ray (XRF) analyzer. A fluorescent X-ray (XRF) analyzer is
equipped with a software which makes it possible to perform
experimental data processing regarding the correlation between the
intensity of the fluorescent X-ray (XRF) of the element A and the
concentration of the element A.
There are no particular restrictions on such software, as long as
the software is that generally used to perform X-ray fluorescence
(XRF) spectroscopy.
For example, there may be employed a software for use in a general
fluorescent X-ray (XRF) analyzer adopting the FP method, such as an
analysis software: "UniQuant 5." Here, one example of the
abovementioned fluorescent X-ray (XRF) analyzer is a full-automatic
wavelength dispersive fluorescent X-ray analyzer (product name:
Axios by Spectris Co., Ltd.).
In order to achieve a bromine (Br) species concentration of not
greater than 400 ppm when measured by the X-ray fluorescence (XRF)
spectroscopy, it is required that a metal compound as a starting
material of the electrode catalyst 1 and a reagent(s) used in each
production step of the electrode catalyst 1 be carefully selected.
Specifically, there may, for example, be used a metal compound that
does not generate bromine (Br) species, as the metal compound
serving as the starting material of the electrode catalyst 1.
Further, there may, for example, be employed a compound(s) that do
not contain bromine (Br) species, as the reagent(s) used in the
production steps of the electrode catalyst 1.
In order to achieve a chlorine (Cl) species concentration of not
greater than 900 ppm when measured by the abovementioned X-ray
fluorescence (XRF) spectroscopy, it is required that a metal
compound as a starting material of the electrode catalyst 1 and
reagents used in production steps of the electrode catalyst be
carefully selected. Specifically, there may, for example, be used a
metal compound that does not generate chlorine (Cl) species, as the
metal compound serving as the starting material of the electrode
catalyst 1. Further, there may, for example, be employed compounds
that do not contain chlorine (Cl) species, as the reagents used in
the production steps of the electrode catalyst 1.
Further, chlorine (Cl) species can be reduced to equal to or less
than 900 ppm by employing the chlorine reduction methods described
later.
A production method of the electrode catalyst 1 includes a step of
producing an electrode catalyst precursor; and a step of washing
such catalyst precursor to meet the condition where the bromine
(Br) species concentration measured by the X-ray fluorescence (XRF)
spectroscopy is not greater than 400 ppm, and the chlorine (Cl)
species concentration measured by the same method is 0 to 900
ppm.
The electrode catalyst precursor of the electrode catalyst 1 is
produced by having the support 2 support the catalyst components
(core part 4, shell part 5) of the electrode catalyst.
There are no particular restrictions on a production method of the
electrode catalyst precursor as long as the method allows the
catalyst components of the electrode catalyst 1 to be supported on
the support 2.
Examples of the production method of the electrode catalyst
precursor include an impregnation method where a solution
containing the catalyst components of the electrode catalyst 1 is
brought into contact with the support 2 to impregnate the support 2
with the catalyst components; a liquid phase reduction method where
a reductant is put into a solution containing the catalyst
components of the electrode catalyst 1; an electrochemical
deposition method such as under-potential deposition (UPD); a
chemical reduction method; a reductive deposition method using
adsorption hydrogen; a surface leaching method of alloy catalyst;
immersion plating; a displacement plating method; a sputtering
method; and a vacuum evaporation method.
Concentration of bromine (Br) species and concentration of chlorine
(Cl) speNext, the concentrations of the bromine (Br) species and
chlorine (Cl) species of the electrode catalyst precursor are
adjusted to meet the condition where the bromine (Br) species
concentration measured by the X-ray fluorescence (XRF) spectroscopy
is not greater than 400 ppm, and the chlorine (Cl) species
concentration measured by the same method is 0 to 900 ppm.
Specifically, there are employed the following chlorine reduction
methods 1 to 3.
Chlorine reduction method 1First step: The first step is to prepare
a first liquid with an electrode catalyst precursor (I) being
dispersed in an ultrapure water. The first liquid is prepared by
adding such electrode catalyst precursor (I) to the ultrapure
water. Here, the electrode catalyst precursor (I) is produced using
a material containing chlorine (Cl) species, and exhibits a
chlorine (Cl) species concentration higher than a predetermined
chlorine (Cl) species concentration when measured by the X-ray
fluorescence (XRF) spectroscopy (e.g. an electrode catalyst
precursor exhibiting a chlorine (Cl) species concentration value
higher than 8,500 ppm or 7,600 ppm, provided that 8,500 ppm or
7,600 ppm is the predetermined chlorine (Cl) species
concentration).
Second step: The second step is to prepare a second liquid with an
electrode catalyst precursor (II) being dispersed in the ultrapure
water. Specifically, the electrode catalyst precursor (I) contained
in the first liquid is filtrated and washed using the ultrapure
water, followed by repeatedly washing the same until a filtrate
obtained after washing has exhibited an electric conductivity .rho.
that is not higher than a predetermined value when measured by a
JIS-standard testing method (JIS K0552) (e.g. not higher than a
value predetermined within a range of 10 to 100 .mu.S/cm). In this
way, there is obtained the electrode catalyst precursor (II) as
well as the second liquid with such electrode catalyst precursor
(II) being dispersed in the ultrapure water.
Chlorine reduction method 2First step: The first step is to retain
a liquid containing an ultrapure water, a reductant and an
electrode catalyst precursor under at least one temperature
predetermined within a range of 20 to 90.degree. C. for a
predetermined retention time. Here, the electrode catalyst
precursor is produced using a material containing chlorine (Cl)
species, and exhibits a chlorine (Cl) species concentration higher
than a predetermined chlorine (Cl) species concentration when
measured by the X-ray fluorescence (XRF) spectroscopy (e.g. an
electrode catalyst precursor exhibiting a chlorine (Cl) species
concentration value higher than 8,500 ppm or 6,000 ppm, provided
that 8,500 ppm or 6,000 ppm is the predetermined chlorine
concentration).
Second step: The second step is to add the ultrapure water to the
liquid obtained in the first step so as to prepare a first liquid
where an electrode catalyst precursor (I) contained in the liquid
obtained in the first step is dispersed in the ultrapure water.
Third step: The third step is to filtrate and wash the electrode
catalyst precursor contained in the first liquid using the
ultrapure water, followed by repeatedly washing the same until a
filtrate obtained after washing has exhibited an electric
conductivity .rho. that is not higher than a predetermined first
value when measured by a JIS-standard testing method (JIS K0552).
In this way, there is now obtained a second liquid where dispersed
in the ultrapure water is the electrode catalyst precursor
contained in the liquid having an electric conductivity .rho. that
is not higher than the predetermined first value.
Fourth step: The fourth step is to dry the second liquid.
Chlorine reduction method 3First step: The first step is to retain
a liquid containing an ultrapure water, a gas having hydrogen and
an electrode catalyst precursor under at least one temperature
predetermined within a range of 20 to 40.degree. C. for a
predetermined retention time. Here, the electrode catalyst
precursor is produced using a material containing chlorine (Cl)
species, and exhibits a chlorine (Cl) species concentration higher
than a predetermined chlorine (Cl) species concentration when
measured by the X-ray fluorescence (XRF) spectroscopy.
The "ultrapure water" used in the chlorine reduction methods 1 to 3
is a type of water exhibiting a specific resistance R of not lower
than 3.0 M.OMEGA.cm, such specific resistance R being represented
by the following general formula (1) (i.e. an inverse number of the
electric conductivity measured by the JIS-standard testing method
(JIS K0552)). Further, it is preferred that the "ultrapure water"
have a water quality equivalent to or clearer than "A3" as defined
in JISK 0557 "Water used for industrial water and wastewater
analysis." [Formula 1] R=1/.rho. (1)
In the above general formula (1), R represents the specific
resistance, and .rho. represents the electric conductivity measured
by the JIS-standard testing method (JIS K0552).
There are no particular restrictions on the ultrapure water, as
long as the water has an electric conductivity that satisfies the
relationship represented by the general formula (1). Examples of
such ultrapure water include an ultrapure water produced using an
ultrapure water system from "Milli-Q series" (by Merck Ltd.); and
an ultrapure water produced using an ultrapure water system from
"Elix UV series" (by Nihon Millipore K.K.).
The chlorine (Cl) species contained in the electrode catalyst
precursor can be reduced by performing any one of the chlorine
reduction methods 1 to 3. Thus can be obtained an electrode
catalyst 1 in which a bromine (Br) species concentration is
adjusted to be not greater than 400 ppm, and a chlorine (Cl)
species concentration is adjusted to be within the range of 0 to
900 ppm when measured by the X-ray fluorescence (XRF)
spectroscopy.
X-ray fluorescence (XRF) spectroscopy The X-ray fluorescence (XRF)
spectroscopy is, for example, performed in the following
manner.
(1) Measurement Device
Full-automatic wavelength dispersive fluorescent X-ray analyzer
Axios (by Spectris Co., Ltd.) (2) Measurement Condition Analysis
software: "UniQuant 5" (Semi-quantitative analysis software
employing FP (four peak method)) XRF measurement chamber
atmosphere: Helium (normal pressure) (3) Measurement Procedure (i)
Placing a sample-containing sample container into an XRF sample
chamber (ii) Replacing an atmosphere in the XRF sample chamber with
helium gas (iii) Setting the measurement condition to "UQ5
application" as a condition required to use the analysis software
"UniQuant 5" and configuring a mode where calculation is performed
in a mode with the main component of the sample being "carbon
(constituent element of support)" and with a sample analysis
result-display format being "element," under a helium gas
atmosphere (normal pressure)
FIGS. 5A and 5B are a schematic view showing preferable embodiments
of a composition for forming gas diffusion electrode containing the
electrode catalyst of this invention; a gas diffusion electrode
produced using such composition for forming gas diffusion
electrode; a membrane-electrode assembly (MEA) having such gas
diffusion electrode; and a fuel cell stack having such
membrane-electrode assembly (MEA).
As for a fuel cell stack S shown in FIG. 5A, each
membrane-electrode assembly (MEA) 400 serves as a one-unit cell,
and the fuel cell stack S is configured by stacking multiple layers
of such one-unit cells.
Particularly, the fuel cell stack S has a membrane-electrode
assembly (MEA) 400 that is equipped with an anode 200a, a cathode
200b and an electrolyte membrane 300 provided between these
electrodes.
More particularly, the fuel cell stack S has a structure where the
membrane-electrode assembly (MEA) 400 is sandwiched between a
separator 100a and a separator 100b.
Described hereunder are the composition for forming gas diffusion
electrode, a gas diffusion electrode 200a, a gas diffusion
electrode 200b and the membrane-electrode assembly (MEA) 400, all
of which serve as members of the fuel cell stack S containing the
electrode catalyst of this invention.
The electrode catalyst 1 can be used as a so-called catalyst ink
component and serve as the composition for forming gas diffusion
electrode in this invention. One feature of the composition for
forming gas diffusion electrode in this invention is that this
composition contains the aforementioned electrode catalyst. The
main components of the composition for forming gas diffusion
electrode are the abovementioned electrode catalyst and an ionomer
solution. The ionomer solution contains water, alcohol and a
polyelectrolyte exhibiting a hydrogen ion conductivity.
A mixing ratio between water and alcohol in the ionomer solution
can be any ratio, as long as it is the kind of ratio capable of
endowing a viscosity suitable for applying the composition for
forming gas diffusion electrode to the electrode. In general, it is
preferred that an alcohol be contained in an amount of 0.1 to 50.0
parts by weight with respect to 100 parts by weight of water.
Further, it is preferred that the alcohol contained in the ionomer
solution be a monohydric alcohol or a polyhydric alcohol. Examples
of a monohydric alcohol include methanol, ethanol, propanol and
butanol. Examples of a polyhydric alcohol include dihydric alcohols
or trihydric alcohols. As a dihydric alcohol, there can be listed,
for example, ethylene glycol, diethylene glycol, tetraethylene
glycol, propylene glycol, 1,3-butanediol and 1,4-butanediol. As a
trihydric alcohol, there may be used glycerin, for example.
Further, the alcohol contained in the ionomer solution may be
either one kind of alcohol or a combination of two or more kinds of
alcohols. Here, the ionomer solution may also be appropriately
allowed to contain an additive(s) such as a surfactant, if
necessary.
For the purpose of dispersing the electrode catalyst, the ionomer
solution contains a hydrogen ion-conductive polyelectrolyte as a
binder component for improving an adhesion to a gas diffusion layer
as a part composing the gas diffusion electrode. Although there are
no particular restrictions on the polyelectrolyte, examples of such
polyelectrolyte include known perfluorocarbon resins having
sulfonate groups and/or carboxylic acid groups. As an easily
obtainable hydrogen ion-conductive polyelectrolyte, there can be
listed, for example, Nafion (registered trademark of Du Pont),
ACIPLEX (registered trademark of Asahi Kasei Chemical Corporation)
and Flemion (registered trademark of ASAHI GLASS Co., Ltd).
The composition for forming gas diffusion electrode can be produced
by mixing, crushing and stirring the electrode catalyst and the
ionomer solution. The composition for forming gas diffusion
electrode may be prepared using crushing and mixing machines such
as a ball mill and/or an ultrasonic disperser. A crushing and
stirring conditions at the time of operating a crushing and mixing
machine can be appropriately determined in accordance with the mode
of the composition for forming gas diffusion electrode.
It is required that the composition of each of the electrode
catalyst, water, alcohol(s) and hydrogen ion-conductive
polyelectrolyte that are contained in the composition for forming
gas diffusion electrode be that capable of achieving a favorable
dispersion state of the electrode catalyst, allowing the electrode
catalyst to be distributed throughout an entire catalyst layer of
the gas diffusion electrode and improving the power generation
performance of the fuel cell.
Particularly, it is preferred that the polyelectrolyte, alcohol(s)
and water be respectively contained in an amount of 0.1 to 2.0
parts by weight, an amount of 0.01 to 2.0 parts by weight and an
amount of 2.0 to 20.0 parts by weight with respect to 1.0 parts by
weight of the electrode catalyst. It is more preferred that the
polyelectrolyte, alcohol(s) and water be respectively contained in
an amount of 0.3 to 1.0 parts by weight, an amount of 0.1 to 2.0
parts by weight and an amount of 5.0 to 6.0 parts by weight with
respect to 1.0 parts by weight of the electrode catalyst. It is
preferred that the composition of each component be within the
abovementioned ranges, because when the composition of each
component is within these ranges, not only a coating film made of
the composition for forming gas diffusion electrode will not be
spread too extensively on the gas diffusion electrode at the time
of forming the film, but the coating film formed of the composition
for forming gas diffusion electrode is also allowed to have an
appropriate and uniform thickness.
Here, the weight of the polyelectrolyte refers to a weight when it
is dry i.e. a weight without a solvent in a polyelectrolyte
solution, whereas the weight of water refers to a weight including
water contained in the polyelectrolyte solution.
The gas diffusion electrode (200a, 200b) of this invention has a
gas diffusion layer 220; and an electrode catalyst layer 240
laminated on at least one surface of the gas diffusion layer 220.
The aforementioned electrode catalyst is contained in the electrode
catalyst layer 240 equipped in the gas diffusion electrode (200a,
200b). The gas diffusion electrode 200 of this invention can be
used as an anode and a cathode.
In FIG. 5A, the gas diffusion electrode 200 on the upper side is
referred to as the anode 200a, whereas the gas diffusion electrode
200 on the lower side is referred to as the cathode 200b for the
sake of convenience.
In the case of the anode 200a, the electrode catalyst layer 240
serves as a layer where a chemical reaction of dissociating a
hydrogen gas sent from the gas diffusion layer 220 into hydrogen
ions takes place due to the function of the electrode catalyst 1
contained in the electrode catalyst layer 240. Further, in the case
of the cathode 200b, the electrode catalyst layer 240 serves as a
layer where a chemical reaction of bonding air (oxygen gas) sent
from the gas diffusion layer 220 and the hydrogen ions that have
traveled from the anode through the electrolyte membrane takes
place due to the function of the electrode catalyst 1 contained in
the electrode catalyst layer 240.
The electrode catalyst layer 240 is formed using the abovementioned
composition for forming gas diffusion electrode. It is preferred
that the electrode catalyst layer 240 have a large surface area
such that the reaction between the electrode catalyst 1 and the
hydrogen gas or air (oxygen gas) sent from the diffusion layer 220
is allowed take place to the fullest extent. Moreover, it is
preferred that the electrode catalyst layer 240 be formed in a
manner such that the electrode catalyst layer 240 has a uniform
thickness as a whole. Although the thickness of the electrode
catalyst layer 240 can be appropriately adjusted and there are no
restrictions on such thickness, it is preferred that the electrode
catalyst layer 240 have a thickness of 2 to 200 .mu.m.
The gas diffusion layer 220 equipped to the gas diffusion electrode
200 serves as a layer provided to diffuse to each of the
corresponding electrode catalyst layers 240 the hydrogen gas
introduced from outside the fuel cell stack S into gas flow
passages that are formed between the separator 100a and the gas
diffusion layer 220a; and the air (oxygen gas) introduced from
outside the fuel cell stack S into gas passages that are formed
between the separator 100b and the gas diffusion layer 220b. In
addition, the gas diffusion layer 220 plays a role of supporting
the electrode catalyst layer 240 to the gas diffusion electrode 200
so as to immobilize the electrode catalyst layer 240 to the surface
of the gas diffusion electrode 220. The gas diffusion layer 220
also plays a role of improving the contact between the electrode
catalyst 1 contained in the electrode catalyst layer 240 and the
hydrogen gas as well as air (oxygen gas).
The gas diffusion layer 220 has a function of favorably passing the
hydrogen gas or air (oxygen gas) supplied from the gas diffusion
layer 220 and then allowing such hydrogen gas or air to arrive at
the electrode catalyst layer 240. For this reason, it is preferred
that the gas diffusion layer 220 have a water-repellent property
such that a pore structure as a microstructure in the gas diffusion
layer 220 will not be blocked by the electrode catalyst 1 and a
water generated at the cathode 200b. Therefore, the gas diffusion
layer 220 has a water repellent component such as polyethylene
terephthalate (PTFE).
There are no particular restrictions on a material(s) that can be
used in the gas diffusion layer 220. That is, there can be employed
a material(s) known to be used in a gas diffusion layer of a fuel
cell electrode. For example, there may be used a carbon paper; or a
material made of a carbon paper as a main raw material and an
auxiliary raw material applied to the carbon paper as the main raw
material, such auxiliary raw material being composed of a carbon
powder as an optional ingredient, an ion-exchange water also as an
optional ingredient and a polyethylene terephthalate dispersion as
a binder. The thickness of the gas diffusion layer can be
appropriately determined based on, for example, the size of a cell
used in a fuel cell. While there are no particular restrictions on
the thickness of the gas diffusion layer, a thin gas diffusion
layer is preferred for the purpose of ensuring a short diffusion
distance of a reactant gas. Meanwhile, since it is required that
the gas diffusion layer also exhibit a mechanical strength at the
time of performing coating and during an assembly process, there is
usually used a gas diffusion layer having a thickness of about 50
to 300 .mu.m, for example.
As for the gas diffusion electrodes 200a and 200b, an intermediate
layer (not shown) may be provided between the gas diffusion layer
220 and the electrode catalyst layer 240. In such case, each of the
gas diffusion electrodes 200a and 200b has a three-layered
structure composed of the gas diffusion layer, the intermediate
layer and the catalyst layer.
A production method of the gas diffusion electrode is described
hereunder.
The production method of the gas diffusion electrode includes a
step of applying the composition for forming gas diffusion
electrode to the gas diffusion layer 220; and a step of forming the
electrode catalyst layer 240 by drying such gas diffusion layer 220
to which the composition for forming gas diffusion electrode has
been applied. The composition for forming gas diffusion electrode
contains the electrode catalyst 1 with the catalyst components
supported on the support and the ionomer solution containing a
hydrogen ion-conductive polyelectrolyte, water and an
alcohol(s).
The important point when applying to the gas diffusion layer 220
the composition for forming gas diffusion electrode is that the
composition for forming gas diffusion electrode is to be
homogeneously applied to the gas diffusion layer 220. As a result
of homogeneously applying the composition for forming gas diffusion
electrode, there can be formed on the gas diffusion layer 220 a
coating film that has a uniform thickness and is made of the
composition for forming gas diffusion electrode. Although an
application quantity of the composition for forming gas diffusion
electrode can be appropriately determined based on a mode of usage
of the fuel cell, it is preferred that the quantity be 0.1 to 0.5
(mg/cm.sup.2) in terms of the amount of an active metal such as
platinum contained in the electrode catalyst layer 240, from the
perspective of a cell performance of a fuel cell having a gas
diffusion electrode.
Next, after applying to the gas diffusion layer 220 the composition
for forming gas diffusion electrode, the coating film of the
composition for forming gas diffusion electrode that has been
applied to the gas diffusion layer 220 is dried so as to form the
electrode catalyst layer 240 on the gas diffusion layer 220. By
heating the gas diffusion layer 220 on which the coating film of
the composition for forming gas diffusion electrode has been
formed, the water and alcohol(s) in the ionomer solution contained
in the composition for forming gas diffusion electrode will be
evaporated and thus disappear from the composition for forming gas
diffusion electrode. As a result, in the step of applying the
composition for forming gas diffusion electrode, the coating film
of the composition for forming gas diffusion electrode that is
formed on the gas diffusion layer 220 becomes the electrode
catalyst layer 240 containing the electrode catalyst and
polyelectrolyte.
The membrane-electrode assembly 400 of this invention (Membrane
Electrode Assembly, abbreviated as MEA hereunder) has the anode
200a and cathode 200b which serve as the gas diffusion electrodes
200 using the electrode catalyst 1; and the electrolyte 300
dividing these electrodes. The membrane-electrode assembly (MEA)
400 can be produced by stacking the anode 200a, the electrolyte 300
and the cathode 200b in an order of anode 200a, electrolyte 300 and
cathode 200b, and then pressure-bonding the same.
As for the fuel cell stack S of this invention, the one-unit cell
(single cell) is established with the separator 100a (anode side)
being attached to an outer side of the anode 200a of the
membrane-electrode assembly (MEA) 400 obtained, and with the
separator 100b (cathode side) being attached to an outer side of
the cathode 200b of the membrane-electrode assembly (MEA) 400,
respectively. Further, the fuel cell stack S is obtained by
integrating such one-unit cells (single cells). Furthermore, a fuel
cell system is completed by attaching a peripheral device(s) to the
fuel cell stack S and assembling the same.
This invention is described in greater detail hereunder with
reference to working examples. However, this invention is not
limited to the following working examples.
Here, the inventors of this invention confirmed that iodine (I)
species was not detected from the catalysts of the working and
comparative examples, when employing the X-ray fluorescence (XRF)
spectroscopy.
Further, unless otherwise noted in the description of each step of
the following production method, these steps were carried out under
a room temperature and in the air.
Working Example 1
The electrode catalyst of this invention was produced through the
following process. The raw materials of the electrode catalyst that
were used in the working examples are as follows. Carbon black
powder: product name "Ketjen Black EC300" (by Ketjen Black
International Co.) Sodium tetrachloropalladate (II) Palladium
nitrate Potassium chloroplatinate
As a support of the electrode catalyst, there was used a carbon
black powder which was dispersed in a water to prepare a dispersion
liquid of 5.0 g/L. An aqueous solution of sodium
tetrachloropalladate (II) (concentration 20% by mass) of 5 mL was
then delivered by drops into and mixed with such dispersion liquid.
An aqueous solution of sodium formate (100 g/L) of 100 mL was
further delivered by drops into a dispersion liquid thus obtained,
followed by taking the insoluble components through filtering and
then washing the taken insoluble components with a pure water.
After performing drying, there was then obtained a palladium
(core)-supported carbon with palladium being supported on carbon
black.
An aqueous solution of copper sulfate of 50 mM was poured into a
three-electrode electrolytic cell. A reasonable amount of the
palladium-supported carbon prepared above was then added to such
three-electrode electrolytic cell, followed by stirring the same
and then allowing the three-electrode electrolytic cell to stand
still. 450 mV (pair reversible hydrogen electrode) was applied to
the working electrode in a resting state such that copper (Cu)
could uniformly coat the palladium of the palladium-supported
carbon. This is defined as a copper-palladium supported carbon.
An aqueous solution of potassium chloroplatinic acid was delivered
by drops into the solution containing the copper-palladium
supported carbon with palladium being coated by copper, the aqueous
solution of potassium chloroplatinic acid containing platinum (Pt)
in an amount two-fold equivalent of the coating copper in terms of
substance amount ratio. In this way, the copper (Cu) of the
copper-palladium supported carbon was replaced with platinum
(Pt).
After filtering a powder of the particles of such platinum
palladium-supported carbon obtained by replacing the copper (Cu) of
the copper-palladium supported carbon with platinum (Pt), without
performing drying, an ultrapure water was used to wash the same in
a wet state due to a filtrate, followed by drying the same at a
temperature of 70.degree. C. Thus, there was obtained an electrode
catalyst precursor 1 {platinum (Pt)-palladium (Pd) supported carbon
(core part: palladium, shell part: platinum)}, to be employed as a
raw material of the electrode catalyst of this invention.
The electrode catalyst precursor 1 was soaked in an aqueous
solution of sodium formate (0.0028M), and retained at a room
temperature for a predetermined period of time. Then, the electrode
catalyst precursor 1 in the aqueous solution of sodium formate was
filtered and washed with ultrapure water. The filtering and washing
with ultrapure water were repeated by a predetermined number of
times. Next, the electrode catalyst washed with the ultrapure
water, was dried at a temperature of 70.degree. C. In this way, the
electrode catalyst of the working example 1, having the loading
amounts of platinum (Pt) and palladium (Pd) and the concentrations
of chlorine (Cl) species and bromine (Br) species shown in Table 1,
was obtained.
The loading amounts (% by weight) of platinum (Pt) and palladium
(Pd) of the electrode catalyst of the working example 1 thus
obtained, were measured by the following method. The electrode
catalyst of the working example 1 was soaked in an aqua regia to
dissolve the metal. Then, carbon as an insoluble component was
removed from the aqua regia. Next, the aqua regina from which the
carbon had been removed was analyzed by ICP analysis.
The results of ICP analysis showed that the loading amounts of
platinum and palladium were respectively 23.8% by mass and 21.9% by
mass.
Working Examples 2 to 3
In a similar manner to the working example 1 except that the time
period of soaking the electrode catalyst precursor 1 in the aqueous
solution of sodium formate (0.0028M) and the number of times of
filtering and washing with ultrapure water were changed, there were
prepared electrode catalysts of the working examples 2 to 3, having
the loading amounts of platinum (Pt) and palladium (Pd) and the
concentrations of chlorine (Cl) species and bromine (Br) species
shown in Table 1.
The obtained electrode catalysts of the working examples 2 to 3
were analyzed by ICP analysis in a similar manner to the working
example 1 to thereby measure the loading amounts of platinum and
palladium.
Working Examples 4 to 5
Except that the concentration of the aqueous solution of sodium
formate in which the electrode catalyst precursor 1 was soaked; the
time period for soaking the same in the aqueous solution; and the
number of times of filtering and washing with ultrapure water were
changed, there were prepared, in a similar manner to the working
example 1, electrode catalysts of the working examples 4 to 5,
having the loading amounts of platinum (Pt) and palladium (Pd) and
the concentrations of chlorine (Cl) species and bromine (Br)
species shown in Table 1. It is to be noted that the concentration
of the aqueous solution of sodium formate used in the working
example 4 is 0.0025M and the one used in the working example 5 is
0.0040M.
Working Example 6
An electrode catalyst precursor 2 having different loading amounts
of platinum (Pt) and palladium (Pd) was employed instead of the
electrode catalyst precursor 1 used in the working example 1. This
electrode catalyst precursor 2 was produced in a similar manner as
the electrode catalyst precursor 1 until the washing and drying
step of the working example 1, in which after the powder of the
particles of the platinum palladium-supported carbon was filtered,
the filtered powder of the particles of the platinum
palladium-supported carbon being in a wet state with a filtrate,
was washed with ultrapure water, and dried at a temperature of
70.degree. C.
Except that when the electrode catalyst precursor 2 was processed
with the aqueous solution of sodium formate, the concentration of
the aqueous solution of sodium formate was rendered to be 0.010M,
and the period of time for soaking the same in the aqueous solution
and the number of times of filtering and washing with ultrapure
water were changed, there was prepared, in a similar manner to the
working example 1, an electrode catalyst of the working example 6,
having the loading amounts of platinum (Pt) and palladium (Pd) and
the concentrations of chlorine (Cl) species and bromine (Br)
species shown in Table 1.
Further, ICP analysis was performed in a similar manner to the
working example 1 to thereby measure the loading amounts of
platinum and palladium.
Working Example 7
An electrode catalyst precursor 3 having different loading amounts
of platinum and palladium, was employed instead of the electrode
catalyst precursor 1 used in the working example 1. This electrode
catalyst precursor 3 was produced in a similar manner to the
production step of the electrode catalyst precursor 1 until the
washing and drying step of the working example 1, in which after
the powder of the particles of the platinum palladium-supported
carbon was filtered, the filtered powder of the particles of the
platinum palladium-supported carbon being in a wet state with a
filtrate, was washed with ultrapure water, and dried at a
temperature of 70.degree. C.
Then, the powder of the electrode catalyst precursor 3 was soaked
in an aqueous solution of oxalic acid (0.3M) instead of the aqueous
solution of sodium formate, and retained at a temperature of
90.degree. C. for a predetermined period of time. Then, the powder
soaked in the aqueous solution of oxalic acid was filtered and
washed with ultrapure water. Next, the powder washed with ultrapure
water, was dried at a temperature of 70.degree. C. In this way, the
electrode catalyst of the working example 7, having the loading
amounts of platinum (Pt) and palladium (Pd) and the concentrations
of chlorine (Cl) species and bromine (Br) species shown in Table 1,
was obtained.
Further, ICP analysis was performed in a similar manner to the
working example 1 to thereby measure the loading amounts of
platinum and palladium.
Working Examples 8 to 9
An electrode catalyst precursor 4 having different loading amounts
of platinum and palladium was employed instead of the electrode
catalyst precursor 1 used in the working example 1. This electrode
catalyst precursor 4 was produced in a similar manner to the
production step of the electrode catalyst precursor 1 until the
washing and drying step of the working example 1, in which after
the powder of the particles of the platinum palladium-supported
carbon was filtered, the filtered powder of the particles of the
platinum palladium-supported carbon, being in a wet state with a
filtrate, was washed with ultrapure water, and dried at a
temperature of 70.degree. C.
Except that when the electrode catalyst precursor 4 was processed
with the aqueous solution of sodium formate, the concentration of
the aqueous solution of sodium formate was rendered to be 0.010M,
and the period of time for soaking the same in the aqueous solution
and the number of times of filtering and washing with ultrapure
water were changed, there were prepared, in a similar manner to the
working example 1, electrode catalysts of the working examples 8 to
9, having the loading amounts of platinum (Pt) and palladium (Pd)
and the concentrations of chlorine (Cl) species and bromine (Br)
species shown in Table 1.
Further, ICP analysis was performed in a similar manner to the
working example 1 to measure the loading amounts of platinum and
palladium.
Comparative Example 1
The electrode catalyst precursor 1 used in the working example 1
was used as it was, without filtering and washing the same by an
aqueous solution of sodium formate or the like.
Comparative Example 2
An electrode catalyst precursor 5 having different loading amounts
of platinum and palladium was employed instead of the electrode
catalyst precursor 1 used in the working example 1. This electrode
catalyst precursor 5 was produced in a similar manner to the
production step of the electrode catalyst precursor 1 until the
washing and drying step of the working example 1, in which after
the powder of the particles of the platinum palladium-supported
carbon was filtered, the filtered powder being in a wet state with
a filtrate, was washed with ultrapure water, and dried at a
temperature of 70.degree. C.
The electrode catalyst precursor 5 thus obtained was, without
filtering and washing the same with an aqueous solution of sodium
formate or the like, was used as it was, for an electrode catalyst
of comparative example 2.
In this way, the electrode catalyst of the comparative example 2,
having the loading amounts of platinum (Pt) and palladium (Pd) and
the concentrations of chlorine (Cl) species and bromine (Br)
species shown in Table 1, was obtained.
Further, ICP analysis was performed in a similar manner to the
working example 1 to thereby measure the loading amounts of
platinum and palladium.
Comparative Example 3
An electrode catalyst precursor 6 having different loading amounts
of platinum and palladium was employed instead of the electrode
catalyst precursor 1 used in the working example 1. This electrode
catalyst precursor 6 was produced in a similar manner to the
production step of the electrode catalyst precursor 1 until the
washing and drying step of the working example 1, in which after
the powder of the particles of the platinum palladium-supported
carbon was filtered, the filtered powder being in a wet state with
a filtrate, was washed with ultrapure water, and dried at a
temperature of 70.degree. C.
Subsequently, the obtained electrode catalyst precursor 6 was
further soaked in an aqueous solution of sodium formate (0.01M),
and retained at a temperature of 90.degree. C. for a predetermined
period of time. Then, the electrode catalyst in the aqueous
solution of sodium formate was filtered and washed with ultrapure
water.
In this way, the electrode catalyst of the comparative example 3,
having the loading amounts of platinum (Pt) and palladium (Pd) and
the concentrations of chlorine (Cl) species and bromine (Br)
species shown in Table 1, was obtained.
Further, ICP analysis was performed in a similar manner to the
working example 1 to thereby measure the loading amounts of
platinum and palladium. Concentrations of bromine (Br) species and
chlorine (Cl) species X-ray fluorescence (XRF) spectrometry was
employed to measure the concentrations of the bromine (Br) species
and chlorine (Cl) species of the electrode catalysts that were
obtained in the working examples 1 to 9, and the comparative
examples 1 to 3. The concentrations of the bromine species and
chlorine species in the electrode catalysts were measured using the
wavelength dispersive fluorescent X-ray analyzer Axios (by Spectris
Co., Ltd.). Specifically, the measurement was carried out through
the following procedure.
A measurement sample of the electrode catalyst was placed in a XRF
sample container equipped in the wavelength dispersive fluorescent
X-ray analyzer. The XRF sample container in which the measurement
sample of the electrode catalyst had been placed was then put into
an XRF sample chamber, followed by replacing an atmosphere in the
XRF sample chamber with a helium gas. Later, fluorescent X-ray
measurement was conducted under the helium gas atmosphere (normal
pressure).
As a software, there was used "UniQuant5" which is an analytic
software for use in wavelength dispersive fluorescent X-ray
analyzer. A measurement condition(s) were set to "UQ5 application"
in accordance with the analytic software "UniQuant5," where
calculation is performed in a mode with the main component of the
measurement sample of the electrode catalyst being "carbon
(constituent element of electrode catalyst support)" and with a
sample analysis result-display format being "element." Measurement
results were analyzed using the analytic software "UniQuant5" such
that the concentrations of bromine (Br) species and chlorine (Cl)
species were able to be calculated. The measurement results are
shown in Table 1.
The catalytic activities of the electrode catalysts produced in the
working examples 1 to 9, and the comparative examples 1 to 3, were
evaluated by a rotating disk electrode method (RDE method). The
catalytic activities of the electrode catalysts were measured by
the rotating disk electrode method (RDE method) in the following
manner.
A powder of each of the electrode catalysts produced in the working
examples 1 to 9 and the comparative examples 1 to 3 was taken by an
amount of about 8.0 mg through measurement, and was put into a
sample bottle together with an ultrapure water of 2.5 mL, followed
by mixing the same while being placed under the influence of an
ultrasonic irradiation, thus producing a slurry (suspension) of the
electrode catalyst. Next, there was prepared a Nafion-ultrapure
water solution by mixing an ultrapure water of 10.0 mL and a 10% by
weight Nafion (registered trademark) dispersion aqueous solution
(product name "DE1020CS" by Wako Chemical Ltd.) of 20 .mu.L in a
different container. The Nafion-ultrapure water solution of 2.5 mL
was slowly poured into the sample bottle containing the slurry
(suspension) of the electrode catalyst, followed by thoroughly
stirring the same at a room temperature for 15 min while under the
influence of an ultrasonic irradiation, thus obtaining a
composition for forming gas diffusion electrode.
FIG. 6 is a schematic diagram showing a schematic configuration of
a rotating disk electrode measuring device D used in the rotating
disk electrode method (RDE method).
As shown in FIG. 6, the rotating disk electrode measuring device D
mainly includes a measuring device cell 10, a reference electrode
(RE) 20, a counter electrode (CE) 30, a rotating disk electrode 40
and an electrolyte solution 60.
An electrode catalyst layer X was formed on the surface of the
rotating disk electrode 40 equipped to the rotating disk electrode
measuring device D. Further, the catalyst of the electrode catalyst
layer X was evaluated by the rotating disk electrode method.
Particularly, there was used a rotating disk electrode measuring
device D (model HSV110 by Hokuto Denko Corp.) employing HClO.sub.4
of 0.1M as the electrolyte 60, an Ag/AgCl saturated electrode as
the reference electrode (RE) 20 and a Pt mesh with Pt black as the
counter electrode (CE) 30.
A method for forming the electrode catalyst layer X on the surface
of the rotating disk electrode 40 is described hereunder.
The composition for forming gas diffusion electrode that had been
produced above was taken by an amount of 10 .mu.L, and was
delivered by drops onto the surface of the clean rotating disk
electrode (made of glassy carbon, diameter 5.0 mm.phi., area 19.6
mm.sup.2). Later, the composition for forming gas diffusion
electrode was spread on the entire surface of the rotating disk
electrode to form a uniform and given thickness, thereby forming on
the surface of the rotating disk electrode a coating film made of
the composition for forming gas diffusion electrode. The coating
film made of the composition for forming gas diffusion electrode
was dried under a temperature of 23.degree. C. and a humidity of
50% RH for 2.5 hours, thus forming the electrode catalyst layer X
on the surface of the rotating disk electrode 40.
Measurements by the rotating disk electrode method include
performing cleaning inside the rotating disk electrode measuring
device; an evaluation of electrochemical surface area (ECSA) prior
to the measurement; an evaluation of electrochemical surface (ECSA)
before and after an oxygen reduction (ORR) current measurement.
In the rotating disk electrode measuring device D, after soaking
the rotating disk electrode 40 in HClO.sub.4 electrolyte solution
60, the electrolyte solution 60 was purged with an argon gas for
not shorter than 30 min. Then, potential scan was performed for 20
cycles under the condition where the scanning potential was set to
be 85.about.1,085 mV vsRHE, and the scanning speed was set to be 50
mv/sec.
Then, potential scan was performed for three cycles under the
condition where the scanning potential was set to be 50.about.1,085
mV vsRHE, and the scanning speed was set to be 20 mV/sec.
After purging the electrolyte solution 60 with an oxygen gas for
not shorter than 15 minutes, a cyclic voltammogram (CV) measurement
was performed for 10 cycles under the condition where the scanning
potential was set to be 135 to 1,085 mV vsRHE, the scanning speed
was set to be 10 mV/sec, and the rotation speed of the rotating
disk electrode 40 was set to be 1,600 rpm. An electrical current
value at a potential of 900 mV vsRHE was recorded. In addition, the
rotation speed of the rotating disk electrode 40 was individually
set to be 400 rpm, 625 rpm, 900 rpm, 1,225 rpm, 2,025 rpm, 2,500
rpm and 3,025 rpm, and an oxygen reduction (ORR) current
measurement was carried out per each cycle. A current measurement
value was defined as an oxygen reduction current value (i).
Finally, the cyclic voltammogram (CV) measurement was performed for
three cycles under the condition where the scanning potential was
set to be 50 to 1,085 mV vsRHE, and the scanning speed was set to
be 20 mV/sec.
The catalytic activity of the electrode catalyst was calculated
using a correction formula of mass transfer which is based on a
Nernst diffusion-layer model as shown by the following general
formula (2), with the aid of the oxygen reduction current value (i)
obtained above and a limiting current value (iL) measured in the
cyclic voltammogram (CV) measurement. The calculation results of
the working examples 1 to 9 and the comparative examples 1 to 3 are
shown in Table 1.
.times..times..times. ##EQU00001## (In the general formula (2), i
represents the oxygen reduction current (ORR current) measurement
value, iL represents the limiting diffusion current measurement
value, ik represents the catalytic activity.)
TABLE-US-00001 TABLE 1 Bromine Chlorine Pt/% Pd/% species species
Working by by concentra- concentra- example mass mass tion/ppm
tion/ppm ik/mA Working 23.8 21.9 200 900 2.51 example 1 Working
23.8 21.9 200 800 2.64 example 2 Working 23.8 21.9 200 600 2.85
example 3 Working 23.8 21.9 200 500 2.61 example 4 Working 23.8
21.9 200 100 2.83 example 5 Working 19.6 24.4 100 0 2.16 example 6
Working 23.5 21.5 100 900 2.20 example 7 Working 23.7 22.0 200 600
2.30 example 8 Working 23.7 22.0 200 500 2.30 example 9 Comparative
23.8 21.9 200 8400 1.90 example 1 Comparative 23.5 21.5 300 6100
1.68 example 2 Comparative 21.0 22.9 500 0 1.74 example 3
From Table 1, it became clear that, compared to the electrode
catalysts obtained in the comparative examples 1 to 3, the
electrode catalysts of the working examples 1 to 9 according to
this invention were able to exhibit a significantly favorable
catalytic activity.
The electrode catalyst of this invention is capable of
demonstrating a sufficient catalytic performance due to the
contents of chlorine (Cl) species and bromine (Br) species being
reduced to the predetermined levels or lower. Accordingly, this
invention is a type of electrode catalyst that can be used not only
in fuel cells, fuel-cell vehicles and electrical equipment
industries such as those related to cellular mobiles, but also in
Ene farms, cogeneration systems or the like. Thus, the electrode
catalyst of this invention shall make contributions to the energy
industries and developments related to environmental
technologies.
* * * * *